Accii III Illation of Driiiiiluix. — -fpham. 847 
of the till that varies from one foot to a few feet in depth near 
Boston, l)ut is sometimes 10 to 15 feet thick on the tops and 
flanks of drumlins in New Hampshire. This upper part of the 
till is comparatively soft and easy to dig, while its main portion 
below is so compact that it must be picked and is far more expen- 
sive in excavating. The probal)le cause of the contrast in hard- 
ness was the pressure of the ice-sheet upon the lower till during 
its accumulation, while the upper till was contained in the ice and 
dropped looselj' at its melting. Occasionally' a thin layer of sand 
or gravel lies between the englacial and subglacial till, as on the 
top of the drumlin named Convent hill in Somerville, Mass., 
where the upper 3 feet of the till, forming the surface, are under- 
lain along an observed distance of several rods l)y a bed of sand 
from 1 to 3 feet thick. 
Subglacial till is further distinguished from that which was 
finally dropped from the departing ice by its smaller rock frag- 
ments, which are mostlv less than two feet in diameter, and 
sometimes consist only of pebbles, cobbles, and small boulders 
not exceeding half this size, though often it also contains large 
boulders: by the glacialh" worn faces of man}' of these stones, 
which are f recpiently marked with striae ; and b}' traces of a pe- 
culiarly bedded structure, in parallelism with the surface. The 
last feature is especiall}' characteristic of the till in drumlins, 
excepting its upper few feet. Although boulders, gravel, sand 
and clay are thoroughly commingled, the deposit is imperfectly 
laminated and tends to separate and crumble into thin flakes. 
This is frequently noticeable in a fresh excavation, but is most 
distinctly seen after a few weeks of exposure. It shows that the 
ice in its passage added new material to the surface of the ground 
moraine, which generally lay undisturbed beneath. 
To a depth that commonly varies from 5 to 1 (I feet on low or 
moderately undulating tracts, but is often 15 to 20 feet or more on 
the drumlins, the color of the till is yellowish gray or buff, while 
at greater depths it is a darker and bluish gray. This difference in 
color is due to progressive weathering, the influence of air and water 
upon the iron contained in the till having changed it in the upper 
part from protoxide combinations to the hydrous sesquioxide. On 
low tracts the weathering of the till is often limited to its compar- 
atively loose englacial portion, but it has generally extended be- 
yond into the subglacial till of the drumlins and lenticular slopes 
